1. Field of the Invention
This invention relates to a zoom lens, and, more particularly, to a zoom lens for a projection zoom lens for a projection type of display apparatus.
2. Description of Related Art
In recent years, attention is directed to a projection type of display apparatus which has a liquid crystal display (LCD) device. A color image formed on the LCD device is magnified and projected onto a screen remote from the LCD device by means of a projection lens.
FIG. 1 schematically shows a practical structure of a video projector.
Referring to FIG. 1, three primary colors into which light rays 1a emanating from a light source 1 are separated by means of color separation prisms 2 and 4 and directed to liquid crystal display (LCD) devices 5, 6 and 7 on which images are displayed, respectively. Light rays la separated by the color separation prisms 2 and 4 are turned at right angles by reflection mirrors 3 and 9, respectively. Light rays 1 a transmitting through the liquid crystal display (LCD) devices 5, 6 and 7 and bearing three mono-color images are composed as a color image by means of color composing prisms 8 and 10 and then, magnified and projected by means of a projection lens 11 onto a screen (not shown) remote from the zoom lens 11.
Because the light rays 1a emanates at a certain divergent angle from the light source 1, and hence marginal light rays incident on each picture elements of the LCD devices 5-7 have large angles of incidence, in order for general LCD devices to prevent or significantly reduce unevenness in contrast and/or quantitative distribution of light, it is usual to use a micro lens array on a side of incidence of the LCD device so as to place micro lenses corresponding in position to picture elements of the LCD device.
In projection type display apparatuses, such as video projectors, which use an LCD device provided with a micro lens array, because a projection lens encounters aggravation of aperture efficiency if causing vignetting, the projection lens have to meet a demand for suitable performance as a telecentric optical system with respect to light rays on the side of the LCD device.
Typically, telecentric projection lenses suitable for projection type display apparatuses comprise three lens groups, two of which on the image end are axially movable relative to each other and relative to the third lens group. Such an telecentric projection lens is known from, for example, Japanese Unexamined Patent Publication No.7-13077. This telecentric projection lens shows a great change rate of spherical aberration during zooming and is still in need of a high degree of design freedom with respect to aberration correction.